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If the term "zoonotic disease" leaves you a little fuzzy, perhaps "avian bird flu" can paint a clearer picture for you. Zoonosis refers to any pathogen which can travel from a vertebrate to a human. These pathogens are well-suited to their host animal, but when they make the transition to humans they often die out because of differences in immunity. In order to survive in their new hosts, the pathogens must undergo an evolutionary adaptation that arises through mutations, which can lead to a heightened ability to spread through human populations. In many cases, these pathogens do not achieve such a potent mutation and a small outbreak will occur in a concentrated area before the disease dies out. However, when a pathogen does make a successful adaptation, the potential for an epidemic emerges.
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